1. Field of the Invention
This invention relates to methods for forming substantially non-crystalline, ultra-high molecular weight polyolefins which may be used as drag reducing agents for improving flow of hydrocarbons through conduits, particularly pipelines.
2. Description of Related Art
Generally speaking, the flow of liquid in a conduit, such as a pipeline, results in frictional energy losses. As a result of this energy loss, the pressure of the liquid in the conduit decreases along the conduit in the direction of the flow. For a conduit of fixed diameter, this pressure drop increases with increasing flow rate. When the flow in the conduit is turbulent (Reynold""s number greater than about 2100), certain high molecular weight polymers can be added to the liquid flowing through the conduit to reduce the frictional energy losses and alter the relationship between pressure drop and flow rate. These polymers are sometimes referred to as drag reducing agents (xe2x80x9cDRAsxe2x80x9d), and they interact with the turbulent flow processes and reduce frictional pressure losses such that the pressure drop for a given flow rate is less, or the flow rate for a given pressure drop is greater. Because DRAs reduce frictional energy losses, increase in the flow capability of pipelines, hoses and other conduits in which liquids flow can be achieved. DRAs can also decrease the cost of pumping fluids, the cost of equipment used to pump fluids, and provide for the use of a smaller pipe diameter for a given flow capacity. Accordingly, an ongoing need exists to form improved drag reducing materials.
Generally, all commercially viable and available petroleum pipeline drag reducing agents are ultrahigh molecular weight polyalphaolefin polymers that are predominately amorphous, or non-crystalline, are highly and randomly branched polymers produced from various alpha olefin monomers. These particular polymers generally have molecular weights in excess of 15,000,000, and may have molecular weights of 30,000,000 or more.
Polyalphaolefin produced from alpha olefin monomers, generally, incorporate monomer ranging from C4 thru C6 monomers. This particular range of alpha olefin monomers has been found to produce the highest quality and most efficacious DRA polymers. These polymers comprise the substantial bulk of today""s commercially available and viable DRA products. In fact, because of the different refining methods utilized by producers of alpha olefin monomers, only one source of alpha olefin monomers (Shell Chemical Company) is recognized by DRA manufacturers as a viable source for producing drag reducing agents. Prior to the present invention, it was not publicly known why this single source of alpha olefin monomers was capable of producing acceptable polymers possessing the desired ultrahigh molecular weight and required amorphous or branched structural characteristics for drag reducing agents. Accordingly, the inventors set out to discover a way to use alpha olefin monomers from other suppliers, e.g., Chevron-Phillips Chemical Company, which was previously recognized as an unacceptable source of alpha olefin monomers for drag reducing agents, for the production of DRAs.
In doing so, the inventors hypothesized that the unacceptable alpha olefin monomers produced by all suppliers other than Shell Chemical Company contain trace and objectionable quantities of internal components associated with the production of these alpha olefin monomers that interfere with the formation of the polyalphaolefin during polymerization of the alpha olefin monomers. It is believed that this interference leads to severe technical and commercial limitations including processing/handling and poorer performing DRA that prevent selection of these alpha olefin monomers for production of DRAs.
Accordingly, most, if not all, commercially viable DRAs are produced from alpha olefin monomers manufactured by Shell Chemical Company. As a result, shortages of commercially available quantities of alpha olefin monomers from Shell Chemical Company for the production of DRAs frequently occur. Therefore, prior to the present invention, there was only one source of alpha olefin monomers for the production of drag reducing agents.
In one aspect, the present invention is directed to an improvement to methods of increasing the flow of hydrocarbons through conduits, particularly viscous crude oil flowing through pipelines. Surprisingly, it has been discovered that a drag reducing agent (DRA) made in accordance with the methods of this invention provides greater flow improvement when added to a hydrocarbon flowing through a conduit than prior methods. Advantageously, such flow improvement can result when the drag reducing agent""s polymer is added to the hydrocarbon at a concentration of as low as 0.25 part per million (ppm) by weight.
In another surprising aspect, it has been discovered that the amount of polymerization catalyst required to produce drag reducing agents is cut in half by isomerizing the olefin monomers prior to polymerization. Therefore, the costs associated with purchasing and storing the polymerization catalyst are reduced.
In certain aspects, the invention also relates to an improvement to methods of producing amorphous, ultra-high molecular weight drag reducing agents having unexpectedly superior drag reduction properties when combined with liquid hydrocarbons, such as viscous crude oil. The improvement comprises isomerizing olefin monomers, and in particular, alpha olefin monomers, prior to polymerization of the olefin monomers to form the polyolefin.
Broadly, one aspect of the invention involves a method of producing an amorphous polyalphaolefin mixture containing an ultra-high molecular weight polyalphaolefin polymer with an inherent viscosity of at least about 10 deciliters per gram and surprisingly superior drag reducing properties when combined with crude oil that is flowing through a pipeline or other conduit. The method preferably includes the steps of isomerizing alpha olefins to form isomerized alpha olefins, contacting a reactant mixture that includes the isomerized alpha olefin monomers with a transition metal catalyst and a co-catalyst to provide an amorphous polyalphaolefin mixture containing an ultra-high molecular weight polyalphaolefin polymer with an inherent viscosity of at least about 10 deciliters per gram and surprisingly superior drag reducing properties when used with viscous crude oil. The polyalphaolefin mixture can be introduced to a pipeline or other conduit having flowing hydrocarbons, such as viscous crude oil. The polyalphaolefin DRA mixture should be introduced in an amount sufficient to increase the flow of the flowing hydrocarbons, preferably at a concentration of from about 1 to 250 ppm by weight, and more preferably from about 5 to 150 ppm by weight.
A specific embodiment of the invention is directed to a method for forming a drag reducing agent comprising a non-crystalline, ultra-high molecular weight polyalphaolefin having an inherent viscosity of at least about 10 deciliters per gram, by isomerizing alpha olefins to form isomerized alpha olefins, contacting the isomerized alpha olefin monomers with a catalyst system that includes a transition metal catalyst and a co-catalyst mixture that includes an alkylaluminoxane co-catalyst; and polymerizing the alpha olefin monomers at a temperature at about or less than about 25xc2x0 C.; wherein, during the polymerization, at least a portion of the isomerized alpha olefin monomers polymerize in the reactant mixture to provide an ultra-high molecular weight polyalphaolefin.
In another specific embodiment of the invention, the polymerization is terminated by adding a xe2x80x9cdeactivatorxe2x80x9d to the reactant mixture after at least a portion of the alpha olefin monomers polymerize in the reactant mixture, to provide an amorphous, ultra-high weight polyalphaolefin. One example of a deactivator is a mixture of isopropyl alcohol and butylated hydroxytoluene.
A variety of alpha olefin monomers are useful in this invention, including homopolymers, copolymers and terpolymers, which, after isomerization, can be present in the reactant mixture in different amounts, alone or in combination. Preferably, these monomers are isomerized and introduced into the reactant mixture at a charge rate of about 4% to 22% based on total weight of the reactant mixture. Charge rate is herein defined as the weight percent of total charge, including one or more components, e.g., solvent, co-catalyst, catalyst, and isomerized alpha olefin monomers. More preferably, the isomerized alpha olefin monomers are present at a charge rate of 4% to 99.5% based on total weight of the reactant mixture.
Examples of alpha olefin monomers that are useful in this invention are co-monomers of 1-hexene and 1-dodecene alpha olefins; or co-monomers of 1-octene and 1-tetradecene alpha olefins in a 1:1 ratio based upon mole weight of the monomers.
A preferred transition metal catalyst is titanium trichloride, which is preferably present in the reactant mixture in an amount of from about 50 to about 1500 parts per million, preferably from about 75 to about 400 parts per million, based on the total weight of all the reactants or components in the reactant mixture.
A further feature of the process for forming a drag reducing agent comprising a non-crystalline, ultra-high molecular weight polyalphaolefin having an inherent viscosity of at least about 10 deciliters per gram is that the reactant mixture may include at least one hydrocarbon solvent such that the isomerized alpha olefin monomers and polyalphaolefin remain substantially dissolved in the hydrocarbon solvent. An additional feature of the process is that the polymerization of the isomerized alpha olefin monomers continues such that the polyalphaolefin is present in the reactant mixture at a concentration of at least about 4 weight percent based upon the weight of the reactant mixture and the polyalphaolefin having an inherent viscosity of at least about 10 deciliters per gram is formed in less than about 24 hours. Another feature of the process is that the polyalphaolefin has an inherent viscosity of at least about 10 deciliters per gram and is amorphous with substantially no crystalline particles. A further feature of the process is that the flow increase is at least about 30% when the polyalphaolefin is present in hexane at a weight concentration of 1 part per million. Another feature of the process is that the catalyst system may include dibutylaluminum chloride and/or diethylaluminum chloride.
In another specific embodiment, the present invention includes a drag reducing agent comprising a non-crystalline, ultra-high molecular weight polyalphaolefin having an inherent viscosity of at least 10 deciliters per gram, formed by isomerizing alpha olefin monomers to form isomerized alpha olefin monomers, contacting the isomerized alpha olefin monomers with a catalyst system in a reactant mixture, wherein the catalyst system includes a transition metal catalyst, such as titanium trichloride, and the co-catalyst mixture includes an alkylaluminoxane co-catalyst, such as methylaluminoxane and isobutylaluminoxane; and polymerizing the isomerized alpha olefin monomers at a temperature at about or less than 60xc2x0 C., preferably less than 40xc2x0 C., wherein during the polymerization, at least a portion of the isomerized alpha olefin monomers polymerize in the reactant mixture to provide a non-crystalline, ultra-high molecular weight polyalphaolefin.
In yet another specific embodiment, the present invention includes a process for reducing drag in a conduit by forming a drag reducing agent comprising a non-crystalline, ultra-high molecular weight polyalphaolefin, by isomerizing alpha olefin monomers to form isomerized alpha olefin monomers, contacting the isomerized alpha olefin monomers with a catalyst system in a reactant mixture, wherein the catalyst system includes a transition metal catalyst and an alkylaluminoxane co-catalyst; polymerizing the isomerized alpha olefin monomers at a temperature at about or less than 60xc2x0 C., preferably less than 40xc2x0 C.; wherein during the polymerization, at least a portion of the isomerized alpha olefin monomers polymerize in the reactant mixture to provide a non-crystalline, ultra-high molecular weight polyalphaolefin having an inherent viscosity of at least 10 deciliters per gram; and introducing the drag reducing agent into the conduit.
In still another aspect of the invention, a halohydrocarbon co-catalyst may be used in conjunction with a transition metal catalyst to form the drag reducing agent. For example, another specific embodiment of the invention is directed to a process for forming a drag reducing agent comprising a non-crystalline, ultra-high molecular weight polyalphaolefin having an inherent viscosity of at least about 10 deciliters per gram. The process includes the steps of isomerizing alpha olefin monomers to form isomerized alpha olefin monomers, contacting the isomerized alpha olefin monomers with a catalyst system in a reactant mixture, wherein the catalyst system includes a transition metal catalyst and a co-catalyst mixture having at least two co-catalysts, wherein one of the co-catalysts preferably is a halohydrocarbon. More preferably, the co-catalyst mixture also includes alkylaluminoxane. The isomerized alpha olefin monomers are polymerized at a temperature at about or less than 60xc2x0 C., wherein during the polymerization, at least a portion of the isomerized alpha olefin monomers polymerize in the reactant mixture to provide a non-crystalline, ultra-high molecular weight polyalphaolefin.
A further feature of the process for forming a drag reducing agent comprising a non-crystalline, ultra-high molecular weight polyalphaolefin having an inherent viscosity of at least about 10 deciliters per gram is that the halohydrocarbon is preferably a chloride containing halohydrocarbon such as ethylene dichloride. Another feature of the process is that the transition metal catalyst is preferably titanium trichloride. An additional feature of the process is that the catalyst system preferably includes an alkylaluminoxane such as methylaluminoxane and/or isobutylaluminoxane.